This invention relates to a method and an apparatus for controlling a resolver having two phase exciting windings and a single phase detecting winding.
A conventional controlling apparatus for a resolver having two phase exciting windings and a single phase detecting winding is shown in the block diagram in FIG. 1. In this apparatus, a ring counter 1 counts a predetermined number of clock pulses CK, and delivers an output signal to a sine wave (SIN) table 2, a cosine wave (COS) table 3, and to a phase difference counter 10. The sine and cosine wave tables are stored within a read only memory (ROM). Upon reception of the output signal, the sine wave table 2 and the cosine wave table 3 deliver a sine wave and a cosine wave to power amplifiers 6 and 7 through D/A converters 4 and 5, respectively. The outputs of the power amplifiers 6 and 7 are applied to the .alpha. phase and .beta. phase windings of a resolver 8 for exciting the same. According to the rotation of the resolver 8, a detecting phase winding of the resolver 8 delivers a signal representing a rotating angle .theta. of the resolver 8. A wave-shaping circuit 9 shapes the output signal of the resolver 8, and a phase-difference counter 10 counts the clock pulses CK for a period corresponding to the phase difference between the output of the ring counter 1 and the output of the wave-shaping circuit 9. The output of the phase-difference counter 10 is applied to a latch circuit 12 that holds the output for a predetermined time and then delivers the output to a CPU (central processing unit) 13. Upon reception of a command signal via an input-output (I/O) circuit 14, the CPU 13 delivers the output of the latch circuit 12 which represents the rotating angle .theta. as an output through an internal bus and the I/O circuit 14.
More specifically, when the .alpha. and .beta. phase windings of the resolver 8 are excited by voltages e.sub.1 sin .omega.t and e.sub.1 cos .omega.t, respectively, an output voltage of e.sub.2 sin (.omega.t-.theta.) is delivered through the detecting winding of the resolver 8. In the above expression, e.sub.1 and e.sub.2 represent amplitudes of the voltages, .omega. represents an angular frequency, t represents time, and .theta. represents a rotating angle of the resolver 8 measured with reference to the .alpha.-phase magnetization axis.
Accordingly, by measuring the phase difference between the .alpha.-phase exciting voltage e.sub.1 sin .omega.t and the detected voltage e.sub.2 sin (.omega.t-.theta.), the rotating angle .theta. of the rotor of the resolver 8 can be determined.
In the arrangement shown in FIG. 1, clock pulses CK are counted by the ring counter 1, a sine wave and a cosine wave are obtained from the SIN/COS tables 2 and 3, and the two phase windings of the resolver 8 are excited by the sine wave and the cosine wave.
The phase-difference counter 10 counts the clock pulses CK from a specific time designated by the ring counter 1 (in this case, a rising edge of sin .omega.t) to another specific time according to the detected voltage (in this case, a rising edge of sin (.omega.t-.theta.)), and delivers an output signal corresponding to .theta. to the CPU 13, which in turn provides output data related to the rotating angle .theta. of the resolver 8.
However, when the above described resolver control apparatus is actually used, a phase delay is caused by the lead wires 101 extending from the control circuit 100 to the resolver 8 shown FIG. 2, and also by the resolver itself. Such a phase delay results in a problem in the execution of the measurement. That is, since the phase-delay property tends to vary according to ambient temperature, obtaining an absolutely precise measurement of the rotating angle of the resolver is made difficult.
More specifically, a phase delay of .phi. ordinarily is attributed to the control circuit 100, starting from the output side of an instruction device (ring counter 1 in FIG. 1) and ending at the input side of a signal processor (phase-difference counter 10 in FIG. 1). Accordingly, the detected signal applied to the phase-difference counter 10 forming the signal processor is expressed as EQU e.sub.3 sin (.omega.t-.theta.-.phi.) EQU =e.sub.3 sin [.omega.t-(.theta.+.phi.)]
wherein e.sub.3 represents the amplitude of the detected signal.
The angle of phase delay .phi. varies with the length of the lead wires and exterior conditions such as ambient precise control thereof is extremely difficult.
As a consequence, the conventional technique for controlling a resolver has the following disadvantages.
(1) A phase delay is caused by the lead wires 101 extending between the control circuit 100 and the resolver 8. Also, the way and devices required for installing the lead wires impair the precision of the measurement.
(2) The resistance of the windings of the resolver 8 varies with the temperature changes and, thus the time constant and the phase delaying property of the same are varied.
(3) The phase-delaying property of the cable fitting devices also varies with temperature changes.
(4) The phase property of a filter (not shown), which is provided for removing higher harmonics from the control circuit 100 and from the detected voltage varies with temperature changes.